1. Field of the Invention
This invention relates to light modulating devices. Still more particularly, this invention relates to constant intensity electron beam addressed nematic phase liquid crystal light valves operated in the reflective mode.
2. Description of the Prior Art
Light modulation devices are most commonly employed as dynamic real time displays. Whatever their form, they perform essentially the same function as a conventional cathode ray tube (CRT) and, consequently, compete for the same market. As a result, light valve design usually attempts to achieve improved resolution, higher brightness, and/or larger display size than that which can be achieved with a CRT.
The class of light valves involved herein is that of the electro-optic dynamic light valve which uses the electro-optic rotation of light polarization between crossed polarizers as a means for light modulation. Two different types are employed at this time. One type is the electron beam addressed light valve in which the electro-optical material is a KD.sub.2 PO.sub.4 crystal, cooled to within a few degrees of its Curie point (minus 50.degree. C.). The TITUS light valve manufactured by the Sodern Company is such a device. The second type of electro-optic dynamic light valve is the slower light beam addressed photoconductor-liquid crystal light valve which may be coupled to a fiber optic face plate CRT. Devices of this latter type have been developed by the Hughes Aircraft Company. Both devices are operated in the reflective mode.
The KD.sub.2 PO.sub.4 light valve is of a somewhat complex construction. The deuterated crystal must be fastened to a cooling stage (in vacuum) in order that the crystal may be operated near its Curie point. The manufacture of the crystal itself is an involved process due to the extremely stringent crystal purity requirements over a large area, usually a few square centimeters. The crystal cell itself has a high capacitance and requires video driving voltages of the order of 150 volts.
The electron addressed liquid crystal light valve of the present invention incorporates the best characteristics of the two prior devices and, at the same time, avoids the less desirable features of these two prior art devices. The advantages of the instant device described herein below are that the liquid crystal cell is directly electron beam addressed, thus bypassing the need for a photo sensor. A liquid crystal cell requires no fiber optic coupling to a CRT face plate. The liquid crystal cell may be driven by a small video voltage (less than ten volts). The response time of the instant liquid crystal cell is more than adequate for TV frame rates. The instant device is also lightweight, requires no cooling devices, is much easier to fabricate, lacks visible raster lines, and possesses memory capability.
It is appropriate at this point to briefly discuss the properties of liquid crystals in general. In liquid or fluid substances, the molecules are typically randomly distributed and oriented throughout the mass. Conversely, in crystalline solids the molecules are typically rigidly oriented and arranged in a specific crystalline structure. Liquid crystals resemble solid crystals in that the molecules of the liquid crystalline substance are regularly oriented in a fashion analagous to but less extensive than the molecular orientation structure in a crystalline solid. It has been observed that many substances exhibit liquid crystalline characteristics only in a relatively narrow temperature range; below this temperature range these substances appear only as crystalline solids, and above the temperature range they appear only as liquids. Liquid crystals are known to appear in at least three different forms: the smectic, nematic, and cholesteric forms. These structural forms are sometimes referred to as mesophases, thereby indicating that they are states of matter intermediate between the liquid and crystal states.
In the smectic structure the molecules are arranged in layers with their major axis approximately parallel to each other and approximately normal to the planes of said layers. The attractive forces between layers are relatively weak so that the layers are free to move in relation to each other. In the nematic structure the major axis of the molecules lie approximately parallel to each other, but the molecules are not organized into different layers as in the smectic structure. In the cholesteric structure the molecules are believed to be arranged in definite layers as in the smectic structure; however, within a given layer, the molecules are believed to be arranged with their major axis approximately parallel in a fashion resembling the structure of the nematic liquid crystals. Because the major axis of the molecules in the cholesteric structure are believed to be parallel to the planes of the layers, the molecular layers are very thin. Prior art references disclose that cholesteric phase liquid crystals respond optically to the application of an electric field. Also, electrically induced phase changes from cholesteric to nematic have been utilized for light modulation.